It is known to use adjunct processors in conjunction with switching systems in order to supplement and enhance the telecommunications services provided by the switching systems. For example, an AT&T AUDIX.RTM. voice messaging system is often used with an AT&T Definity.RTM. private branch exchange (PBX) to provide users with voice messaging capabilities.
To enable e adjunct processor to perform its functions, generally the switching system supplies the adjunct with requisite information, such as control information or detection of call-progress change of state. However, in instances where the switching system is either unable to obtain the information required by the adjunct processor (for example, is unable to detect call-progress signals) or is unable to communicate this information to the adjunct (for example, because the switching system and the adjunct processor do not use the same communication scheme, that is, do not "speak the same language"), the adjunct processor must obtain the requisite information on its own, or do without it.
Internationally, one of the more difficult call-progress changes of state to detect properly is call disconnection at the far end of the call (that is, by the party to the call other than the one trying to do the delecting). Sometimes, no indication of far-end call disconnection is given at all, other than that the call falls silent. In some countries, digital signalling on digital trunks and out-of-band analog signalling on analog trunks are the only used forms of signalling far-end call disconnection. In many other countries, in-band analog signalling, and particularly tone-based signalling, on analog trunks is principally used to signal far-end call disconnection. Hence, switching systems and other equipment designed for use in the former set of countries generally are not readily able to detect far-end call disconnection when put to use in the latter set of countries. Moreover, even equipment designed to detect tone-based signalling may find it difficult to work properly in the latter set of countries, because the signalling may vary from country to county. What is worse, official requirements for such signalling may not be adhered to in many countries, and so the signalling may not even be consistent within a country. The signalling may actually depend on the particular switching system that is serving the far end of the call and generating the signalling, so that the near-end equipment may, in effect, receive different signalling for every call! The magnitude of this problem can be appreciated from the fact that the CCITT (now the ITU) lists over 50 different signals that can be used as far-end call disconnect-indicative signals in the international arena!
Various techniques have been developed over time to detect analog signalling of disconnect on analog links. In the case where no positive indication of call disconnection is given, detection of the fact that a call has fallen silent for a predetermined minimum period of time has normally been used as an indication of call disconnection. This technique has the disadvantage of not being able to distinguish between long pauses in the conversation or other call traffic, and true call disconnection. Moreover, it wastes resources that are used in serving the call after the call has been disconnected but before the long period of call silence that is interpreted as call disconnection expires.
Some switching systems provide a current-level fluctuation on an analog telephone link to signal call disconnect, and circuits have been developed to detect this fluctuation. However, many switching systems do not provide this fluctuation, some provide it only on telephone lines and not on telephone trunks, and such a fluctuation normally cannot be passed through a telephone network. Hence, monitoring, for the presence of a current-level fluctuation as a way to detect remote call disconnect has limited usefulness.
Almost all known switches eventually return an intercept tone, a reorder tone, a dial tone, or some other call-control tone across an analog call path of a call that stays connected after the local party to the call has hung up. Hence, detection of this tone-based signalling at the other end of the call is a good candidate for generally-applicable detection of far-end call disconnect. And, indeed, various arrangements have been developed for this purpose. They are of two basic types.
Energy-detection arrangements use an energy detector to detect a constant energy pattern in signals present at any time on a telephone line. Energy detectors are well known in the art; for example, digital signal processors (DSPs) commonly perform energy detection as part of their functions. The variance in the detected pattern is then compared against a threshold. If the variance falls below the threshold for a predetermined amount of time, this is taken as an indication of the presence of a call-control signal as opposed to call traffic, under the theory that call-control tones on average have a highly regular energy pattern as compared to voice signals. However, this may be an unfounded assumption. The telephone line may be used to carry traffic signals other than voice, such as music or data, that may exhibit energy characteristics very similar to those of control tones. Hence, this approach is error-prone.
To compensate for the problems of energy-detection arrangements, frequency-detection arrangements employ notch (single-frequency passband) filters at the input of the energy detector. The filters filter out all signal frequencies present on the telephone line other than the one or two frequencies that, in theory, constitute the call control tone that indicates far-end call disconnect. Only the selected one or two frequencies are then supplied to the energy detector, which functions as described previously. These arrangements are made error-prone by their dependence on the disconnect-indicative signal having a very precise and constant frequency or frequencies. They are therefore totally unsuitable for use in countries where the disconnect-indicative signal may fluctuate or, worse yet, be any one of a plurality of different signals. Furthermore, a different one of these arrangements must be designed, built, supplied, maintained, etc. for every country or region of interest that uses a different disconnect-indicative signal--an expensive proposition. Finally, the very sharp narrowband filtering required by these arrangements is very expensive when implemented in hardware and very computationally intensive when implemented in software.
Therefore, what the art needs but lacks is a substantially-universal detector of disconnect-indicative tones.